Process for the preparation of metal and metal fluoride products

ABSTRACT

This invention concerns a process for preparing metal and metal fluoride products. A molten mixture containing a metal oxide, a molten metal fluorinating agent and carbon is reacted to produce a gaseous oxide of carbon, a metal fluorinated product derived from the cation of the metal oxide, and a metal derived from the cation of the metal fluorinating agent.

United States Patent Layne et al.

1 June 24, 1975 PROCESS FOR THE PREPARATION OF METAL AND METAL FLUORIDE PRODUCTS Inventors: Gilbert S. Layne; James O. Huml, both of Midland, Mich.

Assignee: The Dow Chemical Company,

Midland, Mich.

Filed: June 23, 1972 Appl. No.: 265,783

Related U.S. Application Data Continuation-in-part of Ser. No. 445, Jan. 2, 1970, abandoned.

Int. Cl..... COlf 7/54; C011) 31/18; C21b 15/00 Field of Search 423/465, 415; 75/28, 29, 75/38, 33, 40

[56] References Cited UNITED STATES PATENTS 3,485,579 12/1969 Vancil ct al. 423/465 Primary E.\'aminer1Edward Stern Attorney, Agent, or FirmBruce M. Kanuch [57] ABSTRACT 2 "Claims, No Drawings PROCESS FOR THE PREPARATION OF METAL AND METAL FLUORTDE PRODUCTS CROSS REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of patent application Ser. No. 445, filed Jan. 2, 1970, now abandoned.

BACKGROUND OF THE INVENTION These are instances where the conversion of metal oxides to metal fluorides may be desirable. For example, it may be desirable to prepare water-soluble fluoride compounds to be employed in the treatment of water supplies and the like. Also, metal fluoride fluxes sometime become contaminated with various metal oxide materials. It may be desirable to reduce the amount of metal oxide in the flux or convert the metal oxide to a metal compound which is more compatible with the flux, for example, another metal fluoride compound.

Several methods have been employed to prepare metal fluorides. The most common method employs hydrogen fluoride as a raw material. This method requires expensive specialized equipment since hydrogen fluoride must be prepared prior to the preparation of the metal fluoride.

The aqueous IHF treatment of oxide is not an effective process for treating all oxides, since with many oxides, hydrates of the fluorinated products are formed, e.g., AlF,-,.3l-l- O, BeF- Xll-l o, ThF,.4I-l- O and the like. This water cannot normally be removed without reformation of substantial quantities of the original oxides. Likewise, in the aqueous HF process several steps are required, i.e. cooling, grinding, reacting, drying and remelting to prepare purified fluorinated products.

On the other hand, the direct treatment of fused salts with gaseous HP is difficult to conduct because (I the corrosiveness of HE requires specialized equipment; (2) the reaction rate and efficiency of HP is low requiring excessive reaction times or large I-IF losses; and (3) the fluorination reaction is usually incomplete leaving substantial quantities of unreacted metal oxides.

Several other processes have also been developed for producing metal fluoride products. The teachings of U.S. Pat. Nos. 2,690,430; 2,877,095 and 2,958,576 are exemplary of other known processes. All of these processes, however, have certain inherent disadvantages associated with them which are objects to be overcome in the instant invention. For example, CF, is an expensive raw material. The process taught in US. Pat. No. 2,690,430 can be employed only to produce a mixture of fluoride and hydroxide or oxide. In the process taught in US. Pat. No. 2,877,095 anions are present which can contaminate the fluoride product. Moreover, aqueous conversion systems usually require large volumes of materials. High wastes are involved and the required dehydration step is often expensive. In another process, it has been taught to react lead fluoride with magnesium oxide to form lead oxide and magnesium fluoride.

The present invention offers the following advantages over those known in the art. First, the process is a solution reaction. This offers a more efficient process which goes substantially to completion. Secondly. metals which are immiscible with the metal fluoride product are produced in the reaction. Further, the use of carbon plus a fluorinating agent allows for the release of oxygen from the oxides at a lower temperature than when the oxide is reduced with carbon or the fluorinating agent is reacted with the oxide. In addition, in the present process, oxygen is removed as a gaseous oxide of carbon which is easier to handle than are oxides of metal. The present process can be employed to fluorinate refractory oxides. Further, the fluorinated product prepared by the present process is usually substantially free of metal oxide impurities.

SUMMARY OF THE INVENTION The present invention comprises reacting a molten mixture containing a metal oxide, carbon and a metal fluoride reactant to produce a gaseous oxide of carbon, i.e., CO and/or CO a metal fluoride product derived from the cation of the metal oxide and a metal product derived from the metal fluoride reactant. The metal product is immiscible with the metal fluoride product. Preferably at least a portion of the reactants are dissolved in a molten salt solvent. The solvent can comprise one or more of the group of unreacted metal fluoride reactant, metal fluoride product or a diluent salt. A metal fluoride reactant is used which is less thermodynamically stable than the desired metal fluoride product and is also miscible with the solution of the metal oxide and molten solvent.

DETAILED DESCRIPTION OF THE PRESENT INVENTION The process of the present invention is theoretically represented by the following reaction:

A0 MF C W CO and/or CO M,, PMF.

In this formula, AO represents the oxide reactant; MF represents the metal fluoride reactant; C is carbon; M is the metal product; AF is the fluoride product; PMF is excess MF, if any, wherein P represents the number of moles of excess MF (i.e., P may be 0); F is fluoride atom; O is oxygen atom.

In order to convert A0 to AF and M to M,,, the following reaction conditions must exist. First, MF must be less thermodynamically stable than AF. Second, if M is in its lowest normal valent state (e.g., FeF- where FeE and FeP both exist), the atom ratio of F/O of the reactants may range from 0.01 to about 10. When operating under the above conditions, excess MP (PMF) can exist and can be employed as a diluent salt. If M is not present in its lowest normal valent state (e.g., FeF instead of FeF the atom ratio of F/O must range from 0.01 to less than 2 times the valence of M to insure the formation of free metallic M. If, however, it is desired to produce reaction products substantially free of ME, then the atom ratio of F/O, where M is in its lowest valent state or not, should be 2 or less.

Preferably, in all reaction systems, the atom ratio of C to 0 should be at least about 1. If this ratio is less than one the removal of oxygen from the reaction system may be incomplete. If the ratio is greater than 1, some free carbon may be present in the reaction products. Depending on the temperature at which the reaction is conducted CO, CO, or mixtures thereof are formed. A i

Reference may be made to standard references to determine the thermodynamic stabilities of the various MF reactants and AP products desired. After determining the stability of AIF, an MP reactant is chosen which is less thermodynamically stable, i.e., is more reactive,

than the stability of AF at the temperature at which the process is run. It is well recogniz'ed that the thermodynamic stability is,to some degreftemperature d'ependent. The following Table'l lists certain of the metal fluok rides which may be produced and/or employed as a reactant" in the practice of' 't'he pr'ese'nt invention and their corresponding Gibbs "Free" Energy computed 'on an equivalent fluoride basis". ln tlie pre'sntinvention, the Gibbs Free Energy of the flu oriiieproduct (AF). computed on an equivalent fluoride basis,-must be less than the Gibbs Free-Energy of the fluorinating agent (MF), computed'on an equivalent fluoride basis, at the temperature of the process. Notall the metal fluoride reactants or products which are employediin the practice of the present invention are listed in Table l. The Gibbs Free Energy valu'esof other fluorides may be calculated, derived rm'm standard references, or extrapolated from the data given in the table. Also, the table does not list' the "Gibbs Free Energy for all temperature ranges. The Gibbs Free Energy at any particular temperature can be computed by known methods and/or extrapolated from 'the data presented herein.

It is preferred to employ a'metal fluorinating agent whichhas aGibb's-Fre 'Energy, computed on an equivalent"fluoride basis; which is as different from the Gibbs Free Energy of the fluorinated=product as possible. In this way. the maximum degree of reaction is obtained.

TABLE I Gibbs Free Energy of Certain Fluoride Compounds on Fluoride Equivalent Basis* Temperature "C taining more than one of said elements and mixtures of these metal oxides.

Specific oxides include, for example. MgO, A1 0 MgAlgO Slog, Llgo, N330. K20, B203, B and TiO ln addition, the less common oxides. such as U0 CeO- ThO- Y O U 0 are also fluorinated according to the instant invention.

The metal fluorinating agent is a metal fluoride whose Gibbs Free Energy of formation (computed on an eqivalent basis), at the reaction temperature, is greater than the Gibbs Free Energy of formation (computed on an eqivalent fluoridebasis) of the fluorinated product derived from the cation of the metal oxide. That is, the fluorinating agent .is less thermodynamically stable than the fluorinated product. Thus, in any particular reaction, the metal fluorinating agent will depend on the particular metal oxide employed. Exemplary of suitable metal fluorinating agents are MHF2- FeF FeF CoF ,,NiF CuF ZnF- AgF, AgF CdF BiFg, SnF-g. SUP. CI'FQ. CI'F HgF-g, SbFa. GaFg, PbFg, mixtures thereof and the like. The fluorides of lead,

iron and copper are preferred because of theirv avail I Carbon may be employed in any convenient solid Compound 27 227 1227" 1727 FeF, 73 69.3 7 61 53.3 47.8 FeF: 78.8 75.5 67.6 60.4 54.8 AgF 44.3 41.5 38.5 35.0 AgF- I 36.4 33.0 25.5 AIF "111.2 l08.0 I BiF 65.3 61.7 v Col-' I 73.9 t .70.6 62.1 CoF 57 54 45.6 CrF 83 79 60.7 CrF, 7 66.7 63.6 HgF 52.3 -MnF 86.5 78.5 73.0 NiFg 73.6 70.0 62.0 54.5 PbFa 73.7 69.8 61.8 53.8 PbF. 50.5 47.5 SbFq 68 64.6 ThF, 1 13.6 109.8

UF. 'l07.1 l03.7 95.2 87.7 UF 79.6 ZnF: 86.1 f 83.6 74.4 ZrF} -l08.7 NaF 130.2 |25.2 112.6 95.9 78.5 MgF l 28 l23.8 -1l3.5 10l.8 88.5 can 140.2 136.1 -1 26.3 -l16.8 -106.3 Na;,AlF., l25.5 l2l -1 10.1 97.4 85.1 LiF -l40.7 136.1 -124.3 I l15.5 100.3 KF l28.8 -123.8 1l1.6 94.2 78.1 BF 89.1 a 88.1 85.7 83.1 80.4 SiF. 93.9 92.2 87-.9 83.6 78.7 TiF -90.5 8 9.l -85.5 8l.8 78 HgF 43.2 39:8 28.3 16.7 5.9

These values are taken fromthe tahlcs found at pages 244-247 of Smithells Metals Reference Book." 4th I Ed. or calculated from other known data.

form. e.g., graphite-coke. charcoal, carbon black. an-

thracite, etc.

'A molten salt solvent, one in which the metal oxide 65 is soluble. can be employed. Included by way of examcon; oxides of Groups IIIBJYB, and-NB, and actinide and lanthanide series of the periodic table. oxides conple are unreacted metalfluorinating agents, the fluorinated product producied andlor diluent salts. The diluent salt is preferably chemically inert to the reactants and products, but may be in some instances reactive to some degree with ingredients of the system. Examples of diluent salts are Nah, lvlgF- Calf-g and Na;,AllF.;.

The process is carried out at atemperatu're which is sufficient to fuse the metal fluoride saltand dissolve at least a portion of the metal'oxide therein. Generally, the temperature necessary for the reaction to proceed is less than that required to reduce the metal oxide with carbon. The minimum temperature is dependent on the temperature required to maintain the reaction system in a molten state (there is some release of gaseous products, e.g., CU and/or C0 Si l etc., depending on the reactants involved).

One important advantage of the present process is that harmful contaminating oxygen is driven off as a gaseous compound.

The A? and l ft, Products can be separated in a number of different Ways including distillation, liquid-liquid (molten) separation, liquid (molten)-solid separation, or solid-solid separation. The separation method employed is determined by the physical characteristics of the reaction products at the temperature chosen to make the separation. This separation may be carried out at any convenient temperature at which the reaction products are stable.

As previously indicated, one of the utilities offered by the present process is to reduce the concentration of metal oxide contaminants contained in metal fluoride mixtures. For example, in the process of US. Pat. No. 3,397,056, a molten metal fluoride (preferably MglF- can become contaminated in the distiller with various oxides, e.g., M 0 MgAl fi as oxygen is inadvertently admitted to the reaction system. Carbon is also often present. it is desirable to reduce the concentration of oxides in the fluoride sait phase remaining in the distiller since their presence increases the melting point of the metal fluoride mixture and also reduces the thermodynamic activity thereof. Carbon may adversely affect the fluidity of the salt. The contaminated metal fluoride is separated from the depleted aluminum source material and treated to adjust the carbon to oxygen atom ratio to at least about one, preferably to about one (e.g., by the addition of C or a metal oxide). The contaminated metal fluoride is then treated by the method defined hereinbefore to convert at least a portion of the metal oxide to a fluorinated product and remove oxygen and carbon as gaseous oxides of carbon. As an example of such treatment, the carbon to oxygen atom ratio in the mix is adjusted to about 1. The fluoride to oxygen atom ratio is adjusted to about 2/l by the addition of l el l. The system is reacted at a temper ature of about l.4-00C. The reaction which takes place theorized as follows;

(in MgF solvent) wherein Fel is the metal fluorinating agent, A1 0, and l /igAl -O, are the metal oxide reactants and MglF FeIF Al? comprise the molten salt solvent. MglF and Alla, are also the fiuorinated products and Fe is the metal product. CG and/or C0 is separated from the reaction mixture such as by venting. Because of density differences, carbon saturated he can be separated from the reaction system by liquid underflow, centrifugation or solidification followed by material classification. AlFg may be separated from the Mgl and the MglF re turned to the distiller for further reaction with an impure aluminum sourcev However, the presence of AIR; is not harmful to the process described in US. Pat. No. 3,397,056 so that both the magnesium fluoride and aluminum fluoride may be returned to the distiller if desired. The teachings ofU.S.l at.l l0. 3,397,056 are specifically incorporated herein by reference.

EXAMPLE 1 A mixture of stoichiometric amounts (based on the reaction MgO C FelFg MgiF Fe C0) of minus mesh powdered MgO, FelF- and C was heated in a graphite crucible to a temperature of about 1,450C and maintained at that temperature under an argon atmosphere for about 2 hours. CO was liberated freely beginning at about 900C and continued throughout the heating. At about 1,300C the entire mixture appeared to be in a molten form. Upon cooling of the reaction mixture, a dense salt phase and a discrete metal phase were found. The conversion of lVlgO to MglF based on iron metal recovered was 103 per cent and based on the salt recovered 99.5 per cent. Freezing point data obtained on the salt phase gave a value of 1 ,250C 1,265C being the accepted value for Mg F X-ray diffraction analysis of the salt phase showed MglF to be the only crystalline phase. Wet chemical analysis of the salt phase showed it contained 0.10 per cent MgO, 0.005 per cent carbon and 0.10 per cent l e. Wet chemical analysis of the metal phase showed 95.2 per cent Fe and 4.9 per cent C.

This example shows the use of the present method to convert an oxide (h/lgO) directly to a fluorinated product (MgF with the production of iron (Fe) and the removal of oxygen as gaseous CO. Because of complete liquid immiscibility the solid Fe and MglF- phases were easily parted after being cooled and solidified.

EXAMPLE 2 To 32.5 grams of magnesium fluoride (MglF- containing about 20 per cent by weight MgAl- QJ, and about 8 per cent by weight carbon, was added 30.5 grams of lFeF (as a metal fluorinating agent) in a graphite crucible. The crucible was placed in, and insulated from, a gas tight quartz shell and induction heated to 1,400C for about three hours. A combustible gas (CO) was released from the reaction mass at about the fusion temperature of l el (-975C) and continued as heating progressed. When the evolution of gas ceased (at -l,4-00C), the heating was stopped and the crucible and contents were cooled to room temperature.

A two-phase system was produced comprising a solid metal and salt phase. These separated easily from one another and from the crucible. The metal phase weighed 13.66 grams and the salt phase weighed 45.97 grams. X-ray diffraction analysis of the salt phase showed Mglr with 5l 5 per cent AlF as the only observable phases. The theorized reaction which toolt place is:

This example shows the conversion of a contaminatingoxide material in a fluoride salt to prepare fluoride salts of Mg and Al, with the removal of oxygen and carbon as gaseous CO. This method can be used to purify the contaminated metal fluoride formed in the process described in US. Pat No. 3,397,056.

EXAMPLE 3 The present invention was employed to produce the commercially important salt, cryolite (Na3AlF(i)i according to the following equation:

the. procedure comprised heating A1 carbon, NaF and FeF in the indicated stoichiometric amounts to 1,2,00C in a graphite crucible for 2 hours. The cooled solidified final products were a dense, slightly offwhite salt phase and an iron phase. They separated easily into 2 large pieces. Conversion was determined as follows:

Based on recovered Fe 96 7: Based on wt. change of salt phase 95.4% Based on wt. loss (CO evolution) 92.4%

produce cryolite.

In still another procedure, NaAlO is substituted for A1 0 and N820 is substituted for NaF to produce cryolite according to the following equation:

The above reactions should be run at a temperature of at least about 900c.

EXAMPLE 4 In this example MgF- containing stoichiometric amounts of carbon and SiO:, is reacted with ZnF- at a temperature of about l,300C to form gaseous CO, SiF and Zn. The active ingredients react according to the equation:

MgF 2ZnF sio 20 sin 2Co 2 2 In this manner highly purified molten MgF is produced since all the other produces of the reaction are gases. MgF functions as a diluent salt in the reaction since it is more thermodynamically stable than both SiF, and ZnF- ZnF- is less thermodynamically stable at 1,300C than SiF What is claimed is:

1. A process which comprises:

a. reacting NaAlO carbon, NaF or Na O, and FeF at a temperature of at least about 900C, said reac tants being provided in an amount to provide a fluoride atom to oxygen atom ratio ranging from about 0.01 to about l0 and a carbon atom to oxy- 'gen atom ratio of at least about 1, thereby to produce products of Na AlF Fe and CO, and

b. separating said products from each other.

2. The process of claim 1 wherein the reactants are provided in an amount sufficient to provide a fluoride atom to oxygen atom ratio of 2 or less.

. UNITED STATES PATENT OFFICE I CERTIFICATE OF CORRECTION PATENT NO. 1 ,8

DATED 1 June 24, 1975 INVENTOR(S) Gilbert S. Layne & James O. Huml It rs certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below: I

' Column 1, line 10, delete "These" and insert There--.

Column 2, line 33, delete "A0 MF C CO and/or C0 Heat M PMF and insert A0 MF C CO and/or C0 Heat AF M PMF Column 4, line 11, after "equivalent" insert --fluoride.

Column 8, line 21, delete "produces" and insert products.

Signed and Sealed this A ttest:

RUTH C. MASON C. Arte-fling O RSHALL DANN vmmissiuner ufParents and Trademarks 

1. A PROCESS WHICH COMPRISES: A. REACTING NAALO2, CARBON, NAF OR NA2O, AND FEF2, AT A TEMPERATURE OF AT LEAST ABOUT 900*C, SAID REACTANTS BEING PROVIDED IN AN AMOUNT TO PROVIDE A FLUORIDE ATOM TO OXYGEN ATOM RATIO RANGING FROM ABOUT 0.01 TO ABOUT 10 AND A CARBON ATOM TO OXYGEN ATOM RATIO OF AT LEAST ABOUT 1, THEREBY TO PRODUCE PRODUCTS OF NA3AIF6, FE AND CO, AND B. SEPARATING SAID PRODUCTS FROM EACH OTHER.
 2. The process of claim 1 wherein the reactants are provided in an amount sufficient to provide a fluoride atom to oxygen atom ratio of 2 or less. 